This invention relates to luminaire louvers. More particularly, this invention relates to louvers that are sized, shaped, and arranged to improve the performance of luminaires.
Luminaires (i.e., lighting units) can be used to provide indirect ambient lighting for interior spaces by directing some or all of their lighting to overhead surfaces. These types of luminaires are widely used in commercial installations where diffuse reflected light is desirable. They are especially common in office spaces, where such lighting is preferred for tasks involving video display terminals.
Luminaires for ambient lighting are commonly suspended from ceilings. They usually have housings that conceal or otherwise shield their lamp(s) from direct view, while directing light upwards through apertures in the top of the luminaire. To minimize glare and maximize visual comfort, these luminaires provide as even a distribution of light (i.e., no bright spots) over as wide an area as possible. Such uniformity of light can be economically attained by using luminaires that (1) are suspended as far as possible from the surface to be illuminated and (2) emit their maximum intensity light at low upward angles. Coincidently, increased setback and wide light distribution also advantageously result in the fewest number of luminaires and the lowest power density (watts per square foot).
However, suspension lengths are often limited, for example, by low ceiling heights, headroom issues, and the general notion that suspended lighting adds clutter to interior spaces. Even in known luminaires with relatively wide light spread distributions, limited suspension lengths often result in undesirable ceiling brightness and poor ceiling uniformity. In such cases, ceiling uniformity is especially compromised when luminaires are widely spaced to lower energy costs. Undesirable ceiling brightness and poor ceiling uniformity can cause reflected glare on display screens, increasing visual fatigue and reducing worker productivity.
Luminaires for ambient lighting also are commonly mounted to furniture systems and low office partitions. Mounting heights for such luminaires typically range from about 48″ above the floor to about 65″ above the floor. These luminaires advantageously eliminate overhead lighting and potentially create a visually clean, uncluttered, and spacious-appearing interior environment. Moreover, these luminaires may also have bottom apertures that provide local supplementary direct lighting for office tasks. This eliminates the need for auxiliary task lights and further reduces energy use. Because these luminaires are generally mounted farther from ceilings than suspended luminaires, they potentially create more diffuse ambient light characterized by lower ceiling luminances, greater uniformity of ceiling brightness, and greater visual comfort.
However, such furniture/partition mounted luminaires (i.e., indirect luminaires mounted below standing eye height) often have a housing with a large height profile to shield their lamps from direct view. Large height profiles can adversely affect the aesthetic appearance of an interior environment and can also adversely impact workstation functionality. For example, a panel-mounted luminaire with a large height profile may prevent a video display terminal from being positioned at the most desirable viewing location.
While some reduction in height profile is possible with shielding devices (e.g., baffle or louver assemblies, described in detail below), shielding devices can also adversely affect the performance of a luminaire and thus diminish the advantages furniture/partition mounted luminaires have over luminaires suspended from ceilings.
Luminaire performance for ambient light applications is determined by luminaire efficiency and the maximum intensity angle. Luminaire efficiency is the percentage of light generated by the luminaire's lamp(s) that is emitted from the luminaire; the closer to 100%, the higher the efficiency. The maximum intensity angle is the angle at which the maximum intensity light is emitted from the luminaire; the lower the angle, the wider the light distribution. Higher performance results from either higher efficiency, lower maximum intensity angle, or preferably both.
Effective shielding devices can contribute to performance by preventing luminaire lamp(s) from being directly viewed while advantageously directing lamp output at low angles that minimize glare (i.e., at angles near but not at or below the viewing angle). However, as mentioned above, shielding devices can also detract from luminaire performance.
For indirect luminaires employing linear type fluorescent lamps (e.g., 1″ diameter T8 or ⅝″ diameter T5 lamps) or long compact (twin-tube) fluorescent lamps, shielding is often performed by a baffle or louver assembly placed above the lamp(s) in much the same manner as a direct or downlight luminaire is fitted with a baffle or louver assembly below its lamp(s). Typically, such baffles or louvers are made of specular or semi-specular metal or metalized plastic fashioned to advantageously redirect light rays to prevent glare.
Baffle assemblies typically have vertical blades arranged transversely (crosswise) to the lamp length. These vertical blades extend between two side members arranged parallel to the length of the lamp. Multiple vertical blades are arranged along the lamp length between the side members to form a series of apertures through which lamp light passes. The spacing of the blades combined with their height and the angle of light reflecting off their surfaces determine the longitudinal shielding angle of the luminaire. Similarly, the distance between the baffle side members, the angle of light reflecting off their surfaces, and the vertical distance at which the lamp is positioned below the top of the baffle sides determine the lateral or transverse shielding angle of the luminaire.
A significant disadvantage of baffle assemblies involves the transverse shielding angle, transverse aperture width, and luminaire height profile. For a given lamp type, indirect luminaires with wide baffle assemblies, which advantageously result in greater overall efficiency with wider light distributions and greater light intensity at lower vertical angles, require the lamp to be located far below the aperture in order to have acceptable lateral shielding. This results in luminaires that are undesirably bulky with noticeably large height profiles, which can compromise the appearance of a space and limit workstation functionality.
Louver assemblies generally combine a series of transverse (baffle) blades with longitudinal blades positioned between the side members and parallel to the lamp length. (As used herein, “transverse blade” and “cross blade” mean the same thing and are interchangeable.) These transverse and longitudinal blades create an array of multiple, usually rectangular, apertures through which lamp light passes. The result is an assembly wherein the spacing of the transverse and longitudinal blades, their respective heights, and the angle of light reflecting off their surfaces determine the longitudinal and transverse shielding angles of the luminaire.
Generally, the vertical position of the lamp from the louver assembly has little to no effect on the shielding angle. Thus, luminaire height profile can be advantageously reduced to little more than the lamp diameter and louver height. To the extent that reduced louver blade spacings allow for reduced louver height without compromising the shielding angle, very low profile luminaires can be advantageously constructed.
Uniform louver assemblies having transverse and longitudinal blades of equal heights, spacings, and surface profiles are very common. They typically are used to construct low-profile, low-brightness luminaires with consistent vertical shielding from all horizontal viewing angles regardless of the position, orientation, and number of lamps (light sources).
Such uniform louver assemblies, however, have two disadvantages. The first disadvantage adversely affects the efficiency of the luminaire. When louver blades are closely spaced to reduce louver height (and thus advantageously reduce the luminaire height profile) while maintaining the shielding angle, the number and total cross-sectional area of louver blades increases, causing the total open aperture area of the louver to accordingly decrease. This increases the interception and reflection of light rays by louver blades. Typically, luminaire louver blades have a surface reflectance of about 85% to 90%, meaning that about 10% to 15% of the light striking the surface is absorbed (i.e., lost). Consequently, the overall light output of the luminaire decreases and the amount of energy (wattage) required to produce a given lighting level increases. The efficiency and overall performance of the luminaire are therefore lower.
The second disadvantage of uniform louver assemblies adversely affects the maximum intensity angle. Normally, light rays entering the louver assembly either emanate directly from the lamp or have been redirected to desirable angles by a luminaire reflector. Louvers therefore should only intercept and redirect those light rays emanating directly from the lamp at undesirable angles (i.e., those light rays that have the potential to cause direct brightness and glare). However, some louver blades intercept light rays that are already directed at desirable angles, while not intercepting light rays directed at undesirable angles. This is especially common with respect to longitudinal louver blades in single-lamp linear fluorescent luminaires. Each time a light ray encounters a louver blade surface, it is redirected at a generally higher angle. Thus, redundant louver reflections cause the luminaire output to become more concentrated and to exit the aperture at higher vertical angles. This reduces the luminaire's ability to output high intensities at low vertical angles (i.e., near the shielding angle) and disadvantageously leads to less light diffusion and reduced surface (e.g., ceiling) uniformity. This, in turn, adversely affects visual comfort and the general appearance of a space.
In view of the forgoing, it would be desirable to be able to provide a louver assembly that improves the performance of luminaires used for indirect ambient lighting.
It would also be desirable to be able to provide a louver assembly that improves luminaire efficiency.
It would further be desirable to be able to provide a louver assembly that produces a wide spread light distribution pattern with maximum light intensities at low vertical angles.